In a redox reaction, both an oxidation and a reduction must occur. The substance that loses electrons is called the reducing agent or reductant. It is oxidized. The species gaining electrons is called the oxidizing agent or oxidant. It is reduced.
CH4 + 2O2 → CO2 + 2H2O
In this given reaction the oxidising agent is oxygen because it oxidises methane to carbon dioxide and oxygen is reduced to water. Therefore, the reducing agent is methane.
